Explanation:

- The Brahmani River is indeed formed by the confluence of the Sankh River and the South Koel River at Vedvyas in Odisha. This makes it the correct option in this context.

- The Pennar River is another major river, but it flows in the Indian state of Andhra Pradesh and has no connection to the Sankh or South Koel Rivers.

- The Subarnarekha River flows through Jharkhand, West Bengal, and Odisha but is not formed by the Sankh and South Koel Rivers.

- The Sabarmati River is in western India, primarily flowing through Gujarat and has no relation to the two mentioned rivers.